Join a sports team. Rugby, football, wrestling, it doesn't matter. Something manly (Dear god do NOT do ultimate frisbee or some ****). You can be terrible at it for the first couple of weeks/months, but after a while you'll get the hang of it. When you are in college, you'll be one of the cool guys who did sports at school and are in shape/busy attracting women. I didn't start doing team sports until I got into college, and I seriously regret it now.
You can be smart and athletic at the same time, you just have to manage your time wisely.